Traumatic brain injury (TBI) is a leading cause of morbidity and mortality that lacks specific therapy, in part because mechanisms that govern secondary brain injury are incompletely understood. Divergent modes of cell death, including necrosis, apoptosis, and caspase-independent programmed cell death have been demonstrated after experimental TBI, however mechanisms that initiate posttraumatic cell death are poorly understood. Tumor necrosis factor receptor (TNFR) family members (i.e., TNFR1, TNFR2, Fas) are transmembrane proteins involved in cell death, survival, and proliferation that are activated as part of an endogenous inflammatory response to TBI. Preliminary data strongly suggest that TNF alpha and Fas receptor, and discreet combinations of TNFR family members, promote caspase-3 activation, cell death, and motor and cognitive dysfunction after controlled cortical impact (CCI) in mice. We propose 3 Specific Aims to test the overall hypothesis that TNFR family members play key roles in the pathogenesis of TBI. Aim 1 will test the hypothesis that inhibition of both TNF alpha and Fas receptor is required to reduce tissue damage and cognitive outcome after CCI. Aim 2 will move downstream of TNF and identify TNFR combinations (TNFR1/Fas vs. TNFR2/Fas) that promote or inhibit cell death and functional recovery. Aim 3 will identify mechanisms of TNFR-mediated cell death by examining activation of caspase-3 and the extrinsic death pathway (i.e., caspase-8, bid, cytochrome C) in TNF/Fas knockout mice. We will use novel mutant mice lacking TNF/Fas, TNFR1/Fas, orTNFR2/Fas;a novel propidium iodide labeling technique that detects acute posttraumatic cell death in the intact mouse brain;and state of the art image analysis and stereologic methods and equipment available in NIH-sponsored Imaging Core facilities. The goal of this revised proposal is to conclusively demonstrate that redundant and opposing effects of multiple TNFR family members integrate to significantly influence cell death, as well as motor and cognitive outcome, after TBI.